1. Field of the Invention
The invention relates to the positioning of a glass sheet relative to cambering and/or other heat treatment tools. It applies to procedures for cambering glass sheets used for the production of motor vehicle windows and according to which the glass sheets heated to beyond their softening point travel flat on a conveyor before being stopped beneath an upper, shaping element against which they are applied and are then released on a lower element, by means of which shaping optionally continues and/or by means of which the glass sheet is transferred to another device, particularly a cooling device by thermal tempering.
2. Background of the Related Art
In the flat cambering methods referred to hereinbefore, the glass sleets are heated in a furnace traversed by the sheets conveyed on a roller bed. Though providing high production rates it is necessary to have high conveying speeds and considerable furnace lengths. Under these conditions, it is not possible to predict the orientation of a glass sheet when it leaves the furnace. Although this impossibility of making a prediction must be understood in relative terms, because the position of the glass sheets is obviously not completely of a random nature, the remaining uncertainty is sufficient to lead to shaping defects due to the appropriate camber not being respected and/or to marks in the glass. This is because the requisite positioning accuracy beneath the upper element is less than 1 mm and is generally approximately 1/10 to a few tenths of a mm.
Different glass sheet positioning methods have been proposed in the past. Thus, it is e.g. known from EP-B-146 442 to use a starting apparatus ensuring a correct initial positioning on entering the furnace. Means are also known for rectifying the position of a glass sheet in or at the outlet from the furnace, the main difficulty being that on approaching the upper element, the glass becomes softer and consequently any action on it may cause optical defects.
In view of the needed optical quality and the tolerance with regards to the camber as a function of the future installation of the glass plate or window, e.g. in a motor vehicle body, it is necessary to have a perfect positioning of the leading edge of a glass sheet, i.e. a correct frontal positioning.
It is known from European patent application EP-A-267 120 to orient the glass sheet by making the leading edge strike against a retractable shutter (or optionally fingers) extended through the conveyor, said stutter holding back the sheet, while the conveyor continues to move it, thus making the sheet assume an alignment with respect to said shutter. However, such an apparatus has difficulty in operating if the leading edge of the sheets is not oriented parallel to the rollers, which is often the case with triangular members, e.g. those needed for padded sides or arm rests in motor vehicles.
To this disadvantage, which can be obviated by a greater sophistication of the retraction means, is added a more fundamental defect, namely that the glass sheet is only referenced relative to the conveyor. However, the Applicants have noted positioning variations of the upper element. Thus, despite its considerable weight and due to the vertical displacements imposed and the high temperatures to which it is exposed, the upper element has a fluctuating position with respect to the vertical position of the conveyor. Here again, it is not a question of a fluctuation of great magnitude, but it is significant in view of the required positioning accuracy.
Moreover, correcting the position of the glass sheet is only sensible to the extent that it is then stopped precisely at the desired moment. In the aforementioned EP-A-267 120, said stoppage is obtained with a regulatable delay by stopping the conveyor after it meets the glass using a detection device of the type described in EP-B-217 708. Therefore the positioning and stoppage of the glass sheets are two independent operations, so that there is a risk that the orientation of a glass sheet will be modified between the action of the shutter and stoppage. Moreover, the stopping of the glass sheet is obtained directly by the stopping of the conveyor. If the upper element has moved with respect to the conveyor, the stepping position is still incorrect.
Finally at the time of shaping, due to the movement of the upper element against a lower element, there is added a shaping treatment by the lower element (continuation of the shaping and/or discharge to a cooling device). If the upper shape or mold moves, the position of the glass on the lower element can no longer be predetermined and this once again leads to glass cambering and/or marking defects.